1. Technical Field
The present application relates to conductor cables, such as those used with equipment for the production of hydrocarbons from subsurface reservoirs.
2. Background Art
Electric submersible pump (ESP) are used in many industries for pumping liquids, such as oil, water, brines, or a combination thereof. For example, in oilfield applications, ESPs may be used to pump fluids (such as hydrocarbons or water) from the wells to the surface. An example of an ESP used in a well is illustrated in FIG. 1.
As shown in FIG. 1, a downhole assembly 10 is disposed in a well 13 penetrating a formation 11. The downhole assembly 10 is lowered in the well 13 via a wireline, tubing, or cable 12. The downhole assembly 10 may include an electric submersible pump (ESP) 14. In this case, the wireline/cable 12 may include an ESP power cable 15. Power transmission from the surface to the downhole ESP 14 is accomplished by the ESP power cable 15. The ESP power cable 15 is specifically designed for downhole oilfield applications.
ESP cables typically transmit between 500V and 20 kV at a wide range of amperages. ESP cable design should take into account the effects of extreme downhole environments, including high temperature (200° C. or higher) and high pressure (25,000 psi or higher). In addition, the downhole wells typically contain various fluids and gases, such as hydrocarbons, water, brines, hydrogen sulfide (H2S). Various well treatment chemicals and additives may also remain in the wells. Therefore, different wells may have different requirements for ESP cable designs, depending on the bottom hole temperature, the pressure, the water/oil ratio, and the aggressiveness and corrosiveness of the local formation fluids.
FIG. 2A illustrates a cross section of an ESP cable design. An ESP cable may include one or more conductor cables. FIG. 2A show that a conductor cable 20 contains a conductor 21 (typically, copper conductors) and an insulation layer 23. The conductor 21 may be optionally coated with a metal alloy 21a to act as an H2S barrier. The insulation layer 23 is bonded to the conductor 21 by an adhesive layer 22. The adhesive 22 may be a solvent-based or water-based adhesive system. The insulation layer 23 can be made of a polymer, such as ethylene-propylene cross-linked polymers. The insulation layer 23 may be coated on the conductor 21 by extrusion methods. Depending on the well conditions that the cable is designed to encounter, the cable can be further coated with several jacketing or barrier layers 24. Examples of barrier layers 24 may include layers made of, for example, fluoropolymers (which may be applied as extrusion layers or tape wraps), lead, oil-resistant cross-linked nitrile rubber, or oil-resistant crosslinked ethylene-propylene diene rubber (EPDM).
Once individual ESP conductor cables 20 are made, one or more of these cables are finally wrapped with a metal armor, as illustrated in FIG. 2B. The metal armor can be galvanized steel, stainless steel, Monel, or others. As shown in FIG. 2B, a finished ESP cable 27 may contain three ESP conductor cables 20 wrapped in a metal armor 29. However, custom configurations can allow for a greater (i.e., >3) or lesser (i.e., <3) number of conductor cables 20 to be included in a finished ESP cable 27. In addition, the finished ESP cable 27 may include capillary tubes, fiber optics, or additional signal or ground wires (shown as 26 in FIG. 2B).
Installation of ESPs and ESP cables in oil wells can require a great deal of specialized equipment. In addition, the installation can require that the well production be halted during the procedures. One skilled in the art would appreciate that any down time in well operations can be very costly. In some locations, particularly on offshore platforms, downtime in production can cost hundreds of thousands of dollars an hour. Therefore, ESP cable failures can be extremely expensive. Accordingly, it is often desired to optimize the cable designs and cable materials to eliminate or minimize ESP cable failures.
In many operations, ESP cables will be reused at least once, if not many times. Repeated installation and removal of the ESP cables can subject them to extreme thermal and pressure cyclings. Such repeated exposures to high-and-low temperatures and pressures may create micro-voids, typically at the interfaces between various layers. Any micro-voids thus formed on the ESP cable could expand when exposed to low pressure. As a result, any micro-void formation may eventually lead to ESP cable degradation and failure. For example, separation between the conductor (shown as 21 in FIG. 2A) and the insulation may lead to electrical discharge. In addition, any separation can provide a pathway for high pressure downhole gas to migrate up the conductor towards the low pressure region at the surface. When high-pressure gas reaches an area of low pressure, it will expand, causing void formation or damage to the insulation. Gas migration caused by this large pressure gradient can cause serious cable damage and lead to a compromised insulation layer and eventual cable failure.
As noted above, the insulation layers are typically coated over ESP cable conductors using water-based or solvent-based adhesives. Such adhesives typically contain 10-75% solids by weight. The liquid adhesive is wiped or sprayed on a conductor, and then it is dried rapidly (generally by hot air) to remove much of the solvent or water immediately prior to coating the insulation layer. The insulation layer is typically applied by feeding the adhesive-coated conductor through an extruder crosshead where a crosslinkable elastomer is applied over the conductor to form the insulation layer.
There are, however, several potential issues with the current approach to ESP cable manufacturing using liquid-based adhesive systems. As alluded to earlier, one issue relates to proper drying of the adhesive. Incomplete drying leaves excessive organic solvent or water trapped between the insulation layer and the conductor. Such trapped solvents or water may lead to void formation. Conversely, over-drying the adhesive may reduce its adhesiveness, leading to compromised bonding. Thus, micro-voids may form in an ESP cable manufactured with a liquid-based adhesive system, if the liquid-based adhesive is insufficiently or completely dried before extrusion of an insulation layer over the adhesive layer. Because the adhesive layer serves to form a strong bond between the surface of the conductor and the insulation layer, maintaining the integrity of this strong bond, therefore, is important to prevent micro-void formations between the conductor and the insulation.
Another issue with liquid-based adhesive systems is thermal stability and longevity of the adhesives. Typically, over long time and/or high temperatures, adhesive systems eventually degrade. As a result, ESP cables removed from high temperature wells often have little or no adhesion between the conductor and insulation due to thermal degradation of the adhesives. The lack of a strong bond between the insulation and conductor can allow corrosive gas to accumulate at the interface, leading to conductor corrosion. This not only decreases the conductivity of the conductor leading to higher conductor heating, but also allows for the ionization of charged gas particles at the interface. The charged gas ions can lead to corona discharge, which will cause damage to the insulation and can also lead to premature cable failure. Therefore, degradation of the adhesives can render these cables not reusable, although the other components of the cable might still be salvageable.
In addition, there are other potential processing, performance, and environmental issues with water-based or solvent-based adhesive systems. During ESP cable manufacturing, it is desirable to create an even and consistent coating and to dry the adhesive to a uniform tackiness. In addition, it is desirable to remove any excess solvent or water.
Performance concerns for the techniques can relate to the upper temperature limits of the adhesive systems. Systems usually begin to experience slow degradation above 150° C. (302° F.) and can degrade rapidly when subjected to downhole environments with temperatures exceeding 200° C. (392° F.).
Finally, environmental concerns relates to evaporation of volatile organic compounds (VOC) from solvent-based adhesive systems. The solvents used in these systems are often toxic. The amount of these toxic chemicals that can be released into the atmosphere is regulated by the Environmental Protection Agency (EPA). To prevent release of toxic solvents into the atmosphere, solvent vapor recovery systems can be used, but with added expense.
Power cables intended for the transmission of medium and high voltages often include a semi-conductive shield layer over the conductor wires. Such semi-conductor layers are often based on ethylene vinyl acetate acrylate (EVA). The use of EVA in such cables have been described in several patents, including U.S. Pat. No. 6,299,978 issued to Sarma et al.; U.S. Pat. Nos. 6,706,791 and 6,525,119 issued to Tsukada et al.; U.S. Pat. Nos. 6,972,099 and 6,402,993 issued to Easter et al.; and U.S. Pat. No. 6,673,448 issued to Gustafsson et al.
However, the EVA-based semi-conductive shield layers may degrade to generate acetic acid at high temperatures (greater than 150° C.). Formation of acetic acid may not be an issue for power cables operating around ambient temperatures. However, formation of acetic acid may be an issue for ESP cables, which may be exposed to temperatures up to 300° C. The formation of acidic compounds at the conductor/insulation interface can lead to corrosion of the conductors, and presence of polar species at the conductor shield/insulation interface may create an electrical stress concentration that can lead to electrical treeing and subsequent micro-void formation, partial discharge, and eventual cable failure.
U.S. Patent Publication No. 2000/5556697A (by Flenniken et al) discloses a semi-conductive power cable shield that uses EP(D)M and ethylene-alkene (EAM) polymers. These polymers provide effective, long-life alternatives to EVA in crosslinkable semiconductive shield applications. However, the compounds described in this patent lack any adhesive properties and would therefore not be acceptable for ESP cable use.
While water-based or solvent-based adhesive systems and EVA-based semi-conductive layer systems have proven useful under certain operating conditions, there remains a need for better adhesive systems that can maintain strong bonding between conductors and insulation in an electric cable, e.g., ESP cable, in extreme downhole environments.